Insulation thickness determination of polymeric power cables [and reply]

Abstract

highly interesting recent paper [l] brought to the A reader's attention the relation which exists between insulation thickness and cavity size in polymeric cables. The author of [l] reaches the conclusion that decreasing the maximum cavity size in a given cable allows an increase in the cable working voltage. A number of assumptions in the aforementioned paper, however, need some more explanation.(1) In the paper, the opinion that ". . . the discharge inception stress represents the critical value for infinitely long life" seems to be adopted. Inception stress depends on the sensitivity of the discharge detection equipment.Recent research, however, by Bruning and coworkers [2, 31 has shown that it is possible to get insulation deterioration even below the so-called inception level. Whether such deterioration is due to minute discharges not detected by conventional methods [4] or to some chemical processes [2,3] is of course still open to question. Voltage values below the inception level do not necessarily guarantee long insulation life. We would appreciate the author's comments on that.(2) In the paper it is also noted that "during an increase in loading and cable temperature, the pressure in the void rises and gas gradually diffuses through the insulation. When load is decreasing and the cable temperature is dropping, the pressure in the void will decrease below atmospheric pressure". Having said that, how the author of [l] relates the behavior of discharges in cavities, and we mean by that the discharge inception/extinction behavior, to the loading conditions? Theories about the extinction of discharges in cavities abound, and there are researchers claiming that an increase in cavity pressure may lead to discharge extinction [5-71, whereas others claim that the continuous consumption of oxygen leads to pressure reduction in the cavity and consequently to discharge extinction [8,9]. Although this subject is not treated extensively in [l], we would welcome the author's comments on that.(3) In another paragraph of his paper, the author claims that ''. . . discharges at small intensity (-2 pC), which will occur when the system is operated at a voltage higher than its rating, are innocuous". My personal experience with polyethylene [lo, 111 is that even small discharges (-1 to 2 pC) at relatively low electric stresses may lead to a transformation of the insulation surface. Such transformations were observed by Mayoux and coworkers [E], who also noted that there is a distinction between transformation and deterioration. Since we think that the aforementioned claim is not undisputed, we would appreciate the author's clarifications of this point.(4) In the Results and Discussion section the author points out that ". . . intervoid influences are so small as to be negligible. Therefore . . . all voids can be ignored except the one for which the discharge inception voltage is the minimum". What is the evidence for that? Recent theoretical investigation [13] has shown that closely coupled cavities in solid dielectrics may...